1. Field of the Invention
The present invention relates to an ophthalmic instrument equipped with an automatic alignment mechanism and also constructed so that measurement is automatically started when the alignment between the instrument main body and an object eye has been completed by the automatic alignment mechanism.
2. Description the Prior Art
Conventionally, an ophthalmic instrument is provided with an automatic alignment mechanism and an automatic measurement starting mechanism for simplifying the operations to them.
The automatic alignment mechanism has an alignment detecting optical system that optically detects an offset quantity .DELTA. in alignment and a drive control unit, such as a pulse motor, which move the instrument main body based on the detection result in a direction in which the detected alignment offset quantity .DELTA. is reduced. The automatic measurement starting mechanism is constructed so that when the measurement error due to the alignment offset quantity .DELTA. has reduced to be negligible, the operation of the drive control unit is stopped and also a measurement start signal is output to the optical measuring section to make the instrument main body start an operation necessary for measurement (e.g., jetting of an air pulse for cornea deformation in a non-contact type tonometer). In other words, assuming that the maximum value of the offset quantity between the optical axis O1 of the optical measuring system and the optical axis Oe of an object eye, which is allowable in obtaining reliable measured values, is .DELTA.O, the measurement start signal is output when the actual offset quantity .DELTA. is less than the allowable limit .DELTA.O.
However, the aforementioned conventional ophthalmic instrument outputs the measurement start signal even when the alignment offset quantity .DELTA. is slightly less than the allowable limit .DELTA.O (e.g., as shown in FIG. 12, even when the optical axis Oe of the object eye is located near the inside of the boundary of a circle with a radius of "ao" representing the allowable limit).
For this reason, in an ophthalmic instrument that has some time lag between the reception of the measurement start signal and the start of an operation necessary for measurement, there are cases where the optical axis Oe of the object eye will be moved from the inside vicinity to the outside of the aforementioned circle with a radius of "ao" during this time lag. For this reason, there is a problem that the measured values will lack reliability.